Its the black rubber line that went into the old exhaust flapper mechanism. If you sit behind the car looking at the exhaust, reach up on the left side of the muffler. Not sure if you installed the exhaust or had it done? If you had it done they might of tucked it away somewhere without plugging it.

I bought a vacuum line plug and stuck it in the line (a golf tee or screw would work). Then zip tied it to make sure it would never pop out.

I had the same problem with mine and this fixed it. Then I just took another zip tie and zip tied the end folded over to where the base of the line came out if the area near the rear fender well.

The hi frequency sound is annoying. Also, there is some rumble inside the cabin that at certain rpm, the coins placed on the coin holder vibrates. I have to take that out.
Other than that, its quite cool. Does this get any louder as this breaks in ??